1. Field of the Invention
The present invention concerns a method to determine a background phase curve in magnetic resonance (MR) image data and a magnetic resonance device that implements such a method. The invention in particular concerns such a method and such a device with which a background phase curve can be determined in a phase-sensitive flow measurement or in an angiography procedure.
2. Description of the Prior Art
Different methods that allow the determination of flow or movement velocities of tissue using imaging MR techniques are known and described, for example by Heinz Morneburg (Ed.), “Bildgebende Systeme für die medizinische Diagnostik” [“Imaging Systems for Medical Diagnostics”], 3rd Edition, 1995, Publicis MCD Verlag, Erlangen. In order to acquire velocity information, for example, magnetic field gradients can be shifted that cause a velocity-dependent contribution to the phase of the spins or, respectively, the magnetic moment, and thus lead to a phase coding of the velocities. Conclusions as to the velocity at the corresponding point of the examination subject can then be drawn from the phase associated with a pixel in the MR image data. Such a method is described in DE 198 36 592 A1, for example.
Background phase effects can impair the precision of such measurements since the background phase of the velocity-dependent phase information is overlaid. Such background phase effects can have various causes. For example, given phase-sensitive flow measurements or phase contrast flow measurements conducted with an MR device, bipolar gradients can be shifted to code a flow of liquid tissues, for example of blood. The temporally variable gradient fields can lead to induction currents and eddy current effects in the coil geometry of the MR device. Even if induction currents can be at least partially accounted for in the computational evaluation of the measured signals, the eddy current effects can lead to a background phase, for example, which background phase overlays the actual velocity information given MR image data that contain a velocity information (and are also called phase images) and can lead to an adulteration of the flow velocity that is determined in this manner. Another source for background phases can exist in gradient nonlinearities. Independent of their origin, the background phase can lead to quantitative adulterations.
Different approaches can be taken in order to determine a background phase curve. One approach is to repeat a measurement with a suitable phantom that is modeled depending on the shape of the examination subject in order to enable the measurement of phase values at stationary points and to thus determine the background phase. However, such a repetition is normally complicated. An additional approach exists in the formation of tables of background phase values. However, since such a table formation can typically take place only for a specific number of sets of measurement parameters, it would be necessary to either correspondingly limit the parameter space that can be used in the measurement or to interpolate between the table values. Time variations of the MR device can lead to the situation that the quality of the correction decreases over time, even under consideration of these limitations.
In J. W. Lankhaar et al., “Correction of phase offset errors in main pulmonary artery flow quantification”, J Magn Reson Imaging 2005; 22(1):73-9, a correction method for correction of background phase effects is described in which points that correspond to essentially stationary tissue are identified in MR image data that represent a slice of the examination subject. A curve of the background phase is determined by an interpolation, depending on the phase values for the stationary pixels. An image-based correction takes place in this way in the sense that the information necessary to determine the background phase is determined from the MR image data themselves that represent the slice. For example, even if a constant phase offset can already theoretically be corrected when at least one stationary pixel can be identified, due to signal noise a greater number and a suitable distribution of stationary pixels in the MR image data are typically required in order to reasonably correct the MR image data. The method described by J. W. Lankhaar et al. thus can not always be implemented for slices in which only a small number of stationary points are present, or cannot always be implemented with acceptable precision.
Due to the increasing importance of flow measurements with volumetric coverage—for example in “4D flow” images in which a velocity information is determined in a three-dimensional space over time—a need exists for methods and devices that allow the curve of the background phase to be determined.